Eight gates (two mitral, two basal, and four superior) simple; four inferior gates fenestrated by arachnoidal irregular threads. Frontal ring square, constricted on the sides by the equatorial ring. Basal ring with four short conical vertical feet, as in the preceding species. Sagittal ring with a vertical horn on the apex. All rings with small scattered thorns. _Dimensions._--Length of the shell 0.11, breadth 0.13. _Habitat._--North Pacific, Station 235, surface. 6. _Microcubus amphispyris_, n. sp. (Pl. 94, fig. 10). Four gates (two mitral and two basal) simple; eight lateral gates (four superior and four inferior) fenestrated by arachnoidal irregular threads. Frontal ring convex, not constricted by the equatorial ring. Basal ring with six short conical divergent feet (two sagittal and four lateral). Sagittal ring with an apical horn. (Resembles some #Spyroidea#.) _Dimensions._--Length of the shell 0.08, breadth 0.12. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. Genus 429. _Octotympanum_,[48] n. gen. _Definition._--#Tympanida# with two bisected horizontal rings converging laterally and connected in the lateral poles of the equatorial plane. Eight large gates. The genus _Octotympanum_ differs from all other Tympanida in the peculiar connection of the two horizontal rings, which here reach the maximum of their growth, converge towards the equator, and come into direct contact on the poles of the equatorial transverse axis. Therefore the shell appears to be composed of three complete vertical meridian rings, the middle of which (the sagittal ring) is free and simple; the two other (subfrontal) rings are grown together at the lateral poles of the equatorial plane; commonly at these poles there arises (on the right and left) a strong lateral spine, and the base of this (or the crossing point of the two touching horizontal rings) corresponds to the remnants of the shortened frontal ring. The crown-shaped shell exhibits therefore eight large gates; two superior (mitral) and two inferior basal gates, two larger anterior (facial) and two larger posterior (occipital) gates. Seen from the lateral poles {1000}only two of the former are visible, and represent the figure of a vertical 8; seen from the sagittal poles, only two of the latter are visible, in the form of a larger horizontal 8. _Octotympanum_ may be derived from _Acrocubus_ by reduction of the lateral frontal ring to a knot (or cross-point), and by maximal extension of the two parallel horizontal rings, which become curved downwards laterally and touch at the lateral poles. 1. _Octotympanum octospinum_, n. sp.